1. Field of the Invention
The present invention relates to a head actuator of a magnetic disk drive.
2. Description of the Related Art
In recent years, magnetic disk drives, which are a type of external storage device for computers, have been made smaller in size and lower in profile. Recently, there have been demands for reducing the power consumption and increasing the recording density of magnetic disk drives. For increasing the recording density of magnetic disk drives, it is indispensable to increase the number of tracks per unit length of the magnetic disks, i.e., to reduce the width of the tracks. Since a magnetic head needs to be positioned over narrow tracks, it is necessary to increase the positioning accuracy of the magnetic head.
Requirements for increasing the positioning accuracy of the magnetic head are as follows:
(1) Vibrations such as residual vibrations of a slider in a servo track write mode should be reduced. PA1 (2) Vibrations of a spindle motor should be reduced. PA1 (3) Vibrations of a head actuator for positioning the magnetic head should be reduced. PA1 (4) The gain of a servo system should be increased to increase the servo bandwidth. PA1 (a) Head actuators for use in general 2.5- or 3.5-inch magnetic disk drives suffer from resonance due to the rigidity of the actuator arm at frequencies of 10 kHz or below. It is difficult to greatly increase this resonant frequency because of various limitations with respect to a yaw angle and power consumption, for example. PA1 (b) The resonant frequency of an actuator translational mode due to the rigidity of the bearings is lower than 10 kHz, e.g., in the range from 4 kHz to 5 kHz, for example. It is difficult to increase the resonant frequency because the bearing rigidity cannot be increased even if the bearing biasing pressure is varied. PA1 Owing to the resonance described above in (a) and (b), the servo bandwidth of the general magnetic disk drives can be increased to at most about 1 kHz. Since the error in following tracks cannot sufficiently be reduced because of this limitation, it has been highly difficult to increase the track pitch. There have been proposed two-stage tracking actuators which incorporate piezoelectric devices for accurately positioning a magnetic head. For example, two piezoelectric devices are disposed one on each side of the actuator arm, and voltages are applied to the piezoelectric devices in such directions as to expand one of the piezoelectric devices and contract the other piezoelectric device. The magnetic head is rotated in the direction of the piezoelectric device to which the voltage is applied to contract the piezoelectric device.
For reducing the vibrations of the head actuator, it is effective to increase the resonant frequency of a translational mode caused by the rigidity of bearings and a structural body such as an arm. Two-stage actuators are highly effective to meet the requirements (3), (4). The present invention is concerned particularly with a twostage tracking actuator.
In general magnetic disk drives, an actuator arm is rotatably mounted on a base, and a load beam (suspension) is fixed at its proximal end to the distal end of the actuator arm. A slider which supports a magnetic head thereon is mounted on the distal end of the load beam. A coil is mounted on the other end of the actuator arm. A magnetic circuit fixed to a base of the magnetic disk device and the coil jointly serve as a voice coil motor. When the coil is energized, forces act on the coil, rotating the actuator arm.
General head actuators suffer from the following problems:
With the conventional two-stage tracking actuators which incorporate piezoelectric devices, however, when a voltage in a direction opposite to the direction of polarization of the piezoelectric device is applied to the piezoelectric device, the piezoelectric device is exposed to a high-temperature atmosphere, or the piezoelectric device is subjected to aging, the piezoelectric device is depolarized, progressively reducing its displacement per unit voltage. Therefore, the piezoelectric device cannot produce a desired stroke after it has been used for a certain long period of time.
Furthermore, since a high voltage such as of about .+-.30 V is required to actuate the piezoelectric device, a circuit for supplying the high voltage is needed, and noise tends to be applied to the signal line due to the high drive voltage. In addition, the conventional actuators which incorporate piezoelectric devices cannot efficiently be manufactured and are expensive to manufacture. Because of these many problems, the conventional actuators which incorporate piezoelectric devices have not been practically available.
There has been proposed a head actuator for moving only a slider a small distance under electromagnetic forces. Since electromagnetic forces generally decrease as the size of the actuator is reduced, a large current will be required to generate drive forces unless the mass of the movable parts is greatly reduced. Actuating only the slider is also disadvantageous from the standpoint of power consumption.
Moreover, it is not easy to manufacture a magnetic circuit. Inasmuch as a magnetic flux generating mechanism is spaced about 1 mm from a head device (transducer), noise may be added to the signal owing to a leakage flux when the head actuator is in operation. Another problem is that since a magnetic attractive force is applied as a drive force to move the slider, the force is not linearly generated in response to a current, making it difficult to effect positioning control in a wide movable range.
The inventor has proposed a two-stage head actuator in an effort to solve the above problems as disclosed in co-pending application Ser. No. 08/728,079. According to the disclosed two-stage head actuator, a drive force for a microactuator is electromagnetically generated, and the movable component of the microactuator is mounted on the tip end of an actuator arm by a criss-cross spring or a hinge.
If the criss-cross spring is used, then the two-stage head actuator can be designed such that the rotational rigidity of the movable component of the microactuator will be low, and at the same time the lateral and vertical translational rigidity thereof will be high. The low rotational rigidity makes it possible to develop a large displacement with a small current. Therefore, only the microactuator is effective to perform a seek mode for a short stroke. The high lateral and vertical translational rigidity increases the servo bandwidth of the actuator, resulting in an increase in the positional accuracy. The electromagnetic motor is also advantageous in that it is highly reliable for a long period of time and inexpensive to manufacture.
However, the two-stage head actuator which uses the criss-cross spring to install the microactuator is problematic in that it cannot easily be manufactured because the criss-cross spring and a head mounting block which supports the criss-cross spring are complex in structure.